Chemical Engineering

Types of Chemical Reactors

Introduction• Reactor is the heart of Chemical Process.• A vessel designed to contain chemical

reactions is called a reactor.• An industrial reactor is a complex

chemical device in which heat transfer, mass transfer, diffusion and friction may occur along with chemical with the provisions of safety and controls

Basic Principle• All chemical processes are centered in a

chemical reactor. The design of a chemical reactor Is the most important factor in determining the overall process economics.

Basics for Design• Reaction Type• Removal/addition of heat• Need for catalyst• Phases involve• The mode of temperature and pressure

control.• Production capacity or flow • Residence time• Contact/mixing between the reactants

Reaction Types• Direct Combination or Synthesis

Reaction

A + B = AB• Chemical Decomposition or Analysis

Reaction

AB = A + B

Reaction Types• Single Displacement or Substitution

Reaction

A + BC = AC + B• Metathesis or Double Displacement

Reaction

AB + CD = CB

In addition to the basic data, include:• A heat and mass transfer characteristics• Physical, chemical and thermodynamic

properties of components taking part in the reaction.

• Corrosion- erosion characteristics of any potential hazard associated with reaction system.

• Reaction Rate

Endothermic/Exothermic Reactions

• “within- heating” describes a process or reaction that absorbs energy in the form of heat.

• Release energy in the form of heat, light, or sound.

• ∆S > 0 • ∆H < 0

Reaction Rate

• Speed at which a chemical reaction proceeds, in terms of amount of product formed or amount of reactant consumed per unit time.

Factors Influencing Reaction Rate

• Concentration• The nature of reaction• Temperature• Pressure• Catalyst

Modeling Principle:

Inputs + Sources = Output + Sink + Accumulations

Basic Reactor Element

• Material Balances• Heat Transfer and Mass Transfer

Material Balances

• Also called mass balance.• Is an application of conservation of mass

to the analysis of physical systems. • The mass that enters a system must, by

conservation of mass, either leave the system or accumulate within the system .

Mass Balance

Mathematically the mass balance for a system without a chemical reaction is as follows

Input = Output + Accumulation

Mass Transfer

• Is the phrase commonly used in engineering for physical processes that involve molecular and convective transport of atoms and molecules within physical system.

• Transfer of mass from high concentration to low concentration.

Heat Transfer

• Is the transition of thermal energy from a heated item to a cooler item.

• Transfer of Thermal Energy

Modes Of Heat Transfer

• jacket,

• internal coils,

• external heat exchanger,

• cooling by vapor phase condensation

• fired heater.

Reactor Types• They can be classified according to the;1. Mode of operation2. End use application3. No of Phases4. A catalyst is used

Classification by Mode of Operation

• Batch Reactors• Continuous reactors• Semi-batch reactors

Batch Reactor• A “batch” of reactants is introduced into the

reactor operated at the desired conditions until the target conversion is reached.

• Batch reactors are typically tanks in which stirring of the reactants is achieved using internal impellers, gas bubbles, or a pump-around loop where a fraction of the reactants is removed and externally recirculated back to the reactor.

Batch Reactors• Temperature is regulated via internal

cooling surfaces (such as coils or tubes), jackets, reflux condensers, or pump-around loop that passes through an exchanger.

• Batch processes are suited to small production rates, too long reaction times, to achieve desired selectivity, and for flexibility in campaigning different products

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Applications of Batch reactor• Fermentation of beverage products• Waste water treatment

Continuous Reactors• Reactants are added and products

removed continuously at a constant mass flow rate. Large daily production rates are mostly conducted in continuous equipment.

Continuous Reactors• CSTR• Plug Flow Reactor• Tubular flow reactor

CSTR• A continuous stirred tank reactor (CSTR)

is a vessel to which reactants are added and products removed while the contents within the vessel are vigorously stirred using internal agitation or by internally (or externally) recycling the contents.

• CSTRs may be employed in series or in parallel.

CSTR• Residence time – average amount of time a

discrete quantity of reagents spend inside the tank

• Residence time = volumetric flow rate volume of the tank

• At steady state, the flow rate in must be equal the mass flow rate out.

CSTR Applications• Continuous stirred-tank reactors are most

commonly used in industrial processing, primarily in homogeneous liquid-phase flow reactions, where constant agitation is required. They may be used by themselves, in series, or in a battery.

• Fermentors are CSTRs used in biological processes in many industries, such as brewing, antibiotics, and waste treatment. In fermentors, large molecules are broken down into smaller molecules, with alcohol produced as a by-product.

Advantages/Disadvantages of CSTR• Good temperature control is easily

maintained• Cheap to construct• Reactor has large heat capacity• Interior of reactor is easily accessedDisadvantage:• Conversion of reactant to product per

volume of reactor is small compared to other flow reactors

Plug Flow Reactor

Plug flow, or tubular, reactors consist of a hollow pipe or tube through which reactants flow. Pictured below is a plug flow reactor in the form of a tube wrapped around an acrylic mold which is encased in a tank. Water at a controlled temperature is circulated through the tank to maintain constant reactant temperature.

Plug Flow Reactor

•Reagents may be introduced into the reactor’s inlet•All calculations performed with PFR’s assume no

upstream or downstream mixing. •Has a higher efficiency than a CSTR at the same value

Schematic Diagram of Plug Flow Reactor

Applications of Plug flow reactor

• Plug flow reactors have a wide variety of applications in either gas or liquid phase systems. Common industrial uses of tubular reactors are in gasoline production, oil cracking, synthesis of ammonia from its elements, and the oxidation of sulfur dioxide to sulfur trioxide.

Tubular Flow Reactor• A tubular flow reactor (TFR) is a tube (or pipe)

through which reactants flow and are converted to product.

• The TFR may have a varying diameter along the flow path.

• In such a reactor, there is a continuous gradient (in contrast to the stepped gradient characteristic of a CSTR-inseries battery) of concentration in the direction of flow.

• Several tubular reactors in series or in parallel may also be used. Both horizontal and vertical orientations are common

Tubular Flow Reactor

Chemical reactions take place in a stream of gas that carries reactants from the inlet to the outlet

The catalysts are in tubes Uniform loading is ensured by using special equipment that charges the same amount of catalyst to each tube at a definite rate.

Semi Batch Reactor• Some of the reactants are loaded into the reactor, and

the rest of the reactants are fed gradually. Alternatively, one reactant is loaded into the reactor, and the other reactant is fed continuously.

• Once the reactor is full, it may be operated in a batch mode to complete the reaction. Semi-batch reactors are especially favored when there are large heat effects and heat-transfer capability is limited. Exothermic reactions may be slowed down and endothermic reactions controlled by limiting reactant concentration.

Semi Batch reactors

• In bioreactors, the reactant concentration may be limited to minimize toxicity.

• Other situations that may call for semibatch reactors include control of undesirable by-products or when one of the reactants is a gas of limited solubility that is fed continuously at the dissolution rate.

Classification By End Use

• Chemical reactors are typically used for the synthesis of chemical intermediates for a variety of specialty (e.g., agricultural, pharmaceutical) or commodity (e.g., raw materials for polymers) applications.

Classification by End use

• Polymerization Reactors• Bio-reactors• Electrochemical Reactors

Polymerization Reactors

• Polymerization reactors convert raw materials to polymers having a specific molecular weight and functionality. The difference between polymerization and chemical reactors is artificially based on the size of the molecule produced.

Bio Reactors

• Bioreactors utilize (often genetically manipulated) organisms to catalyze biotransformations either aerobically (in the presence of air) or an-aerobically (without air present).

Electrochemical reactors

• Electrochemical reactors use electricity to drive desired reactions.

• Examples include synthesis of Na metal from NaCl and Al from bauxite ore.

• A variety of reactor types are employed for specialty materials synthesis applications (e.g., electronic, defense, and other).

Classification by Phase

• Despite the generic classification by operating mode, reactors are designed to accommodate the reactant phases and provide optimal conditions for reaction.

• Reactants may be fluid(s) or solid(s), and as such, several reactor types have been developed.

• Single phase reactors are typically gas- (or plasma- ) or liquid-phase reactors.

• Two-phase reactors may be gas-liquid, liquid-liquid, gas-solid, or liquid-solid reactors.

Classification by phase

• Multiphase reactors typically have more than two phases present. The most common type of multiphase reactor is a gas-liquid-solid reactor; however, liquid-liquid-solid reactors are also used. The classification by phases will be used to develop the contents of this section.

Classification by Phase• In addition, a reactor may perform a function other than

reaction alone. Multifunctional reactors may provide both reaction and mass transfer (e.g., reactive distillation, reactive crystallization, reactive membranes, etc.), or reaction and heat transfer.

• This coupling of functions within the reactor inevitably leads to additional operating constraints on one or the other function. Multifunctional reactors are often discussed in the context of process intensification.

• The primary driver for multifunctional reactors is functional synergy and equipment cost savings.

CATALYSIS

CATALYSIS

• It is the acceleration of chemical reaction by means of substance called catalyst.

Principles of Catalysis:

∙Typical mechanism: A + C → AC (1) B + AC → ABC (2) ABC → CD (3) CD → C + D (4)

•Catalysis and reaction energetic.

What is Phase?

Two Types of Catalyst:

∙Homogeneous∙Heterogeneous

Homogeneous

• the catalyst in the same phase as the reactants.

Heterogeneous

• Involves the use of a catalyst in a different phase from the reactants.

How the heterogeneous catalyst works?

•Adsorption•Active Sites•Desorption

Adsorption

•Is where something sticks to a surface.

Active Sites

• Is a part of the surface which is particularly good at adsorbing things and helping them to react.

Desorption

• means that the product molecules break away.

Kinds of Catalyst• Strong Acids• Base Catalysis• Metal oxides, Sulfides, and Hydrides• Metal and Alloys • Transition-metal Organometallic

Catalysts

Strong Acids

• Is an acid that ionizes completely in an aqueous solution

Base Catalysis

• Is most commonly thought of as an aqueous substance that can accept protons.

• Base the chemical opposite of acids.• Often referred to as an alkali if OH−

ions are involved.

Metal Oxides

• Form a transition between acid/base and metal catalysts.

Metal and Alloy

• Metal is a chemical elements whose atoms readily lose electrons to form positive ions (cations), and form metallic bonds between other metal atoms and ionic bonds between nonmetal atoms.

• The principal industrial metallic catalyst, are found in periodic group VII

Transition-metal Organometallic Catalysts

•More effective hydrogenation than are metals such as platinum.

Fluid and Solid Catalysis

• Multitubular reactors• Fluidized beds• Fixed Bed • Spray Tower• Two-Phase Flow

Multitubular reactors

• These reactors are shell-and-tube configuration and have catalyst in the tubes.

Multi tubular Reactor

Fluidized Bed

• Device that can be used to carry out a variety of multiphase chemical reactions.

• A catalyst possibly shaped as tiny spheres.

Fluidized Bed Reactor

Fixed Bed • Fixed bed reactor is a

cylindrical tube, randomly filled with catalyst particles, which may be spheres or cylindrical pellets.

Fixed Bed Reactor

SPRAY TOWER

• Are a form of pollution control technology.

• Consist of empty cylindrical vessels made of steel or plastic and nozzles that spray liquid into the vessels

Two types of Spray Towers:

1.Cocurrent Flow -are smaller than countercurrent-flow

spray towers

2.Crosscurrent Flow - the gas and liquid flow in directions

perpendicular to each other.

Two-Phase Flow

• occurs in a system containing gas and liquid with a meniscus separating the two phases.

Two-phase flow may be classified according to the phases involved as:

• gas-solid mixture• gas-liquid mixture• liquid-solid mixture• two-immiscible-liquids mixture

Diesel Hydrotreator reactor

Hydrotreating

• Hydrotreating is an established refinery process for reducing sulphur, nitrogen and aromatics while enhancing cetane number, density and smoke point. The refining industry’s efforts to meet the global trend for more-stringent clean fuels specifications, the growing demand for transportation fuels and the shift toward diesel mean that hydrotreating has become an increasingly important refinery process in recent years.